In the art of mass transfer for selectively separating at least one component from a mixture of at least two constituents as for example in distillation columns of an air separation plant, an upwardly flowing vapor or gas stream is typically contacted on a substantially horizontally aligned contacting tray with a generally downwardly flowing liquid stream. In a conventional distillation process, such contacting permits the upwardly flowing vapor or gas stream to become selectively enriched with the lighter components of the mixture, i.e., those components with relatively high volatilities, while the generally downwardly flowing liquid stream becomes selectively enriched with the heavier component of relatively low volatilities. A variety of types of gas-liquid contacting trays have been conventionally employed in the above described mass transfer operations, to effect intimate contacting between the respective gas and liquid phases. Features common to the various types of contacting trays are downcomers, weirs and the active area, briefly described in the paragraphs that follow.
The area of the tray deck which contains the apertures in conventional vapor-liquid contact trays is often referred to as the “active area” of the contacting tray because the vapor-liquid interaction occurs above the apertures in the tray. In general, the liquid and vapor handling capacity of the tray is limited by the available active area of the tray as well as the area of the downcomer. If the amount of descending liquid or ascending vapor exceeds the tray capacity, flooding of the tray will occur as either the entrained liquid is unable to adequately disengage from the associated vapor stream or the vapor is unable to disengage from the liquid stream.
A weir is also used on most vapor-liquid contacting trays to cause liquid to accumulate on the top surface of the tray for enhanced interaction with the vapor bubbling through the apertures in the tray deck. The vapor and liquid interaction on the tray desirably causes a froth to build up on the tray. Because the liquid phase remains substantially continuous in the froth, the vapor and liquid interaction continues in the froth and results in greater mass transfer efficiencies.
Downcomers are conventionally provided in combination with the vapor-liquid contacting trays to provide a passage through which liquid is discharged from one tray to an underlying tray. In single pass trays, the downcomers are provided at opposite ends of vertically adjacent trays so the liquid must flow completely across one tray before it enters the downcomer for passage to the next lower tray. The liquid on the lower tray then flows in the opposite direction across the tray and enters another downcomer. This back-and-forth flow pattern is repeated as the liquid descends through the portion of the column containing the vapor-liquid contact trays. In many two-pass trays, the descending liquid is split into two streams which travel in opposite directions on each contacting tray. A center downcomer is usually provided on alternate trays while two end downcomers are placed at opposite ends of vertically adjacent trays to provide the two pass flow pattern. Finally, multiple pass contacting trays have also been utilized and are typically configured to include a plurality of chordal downcomers to reduce weir loading.
High pressure distillation systems often require high liquid rate contacting trays. For example, contacting trays operating at very high liquid rates must have large liquid handling capability, which in turn requires proper sizing and orientation of the downcomers on the contacting trays. In other words, the efficiency of mass transfer of a given component between these phases in high liquid rate distillation systems is determined largely by the configuration of the downcomers on the contacting trays.
In a well-known classic study by W. K. Lewis in 1936, it was found that the mass transfer efficiency of vapor-liquid contacting trays could be maximized by bringing an unmixed vapor into contact with liquid flows across each successive tray in the same direction (Case 2). The Case 2 is referred to as a parallel flow, which, as used herein, refers to liquid flows on vertical adjacent or successive trays rather than to liquid flows on a single tray. Lewis' Case 2 ensures that the driving force for mass transfer on a given tray is nearly the same regardless of where that mass transfer occurs on the tray. Because of this, substantial increases in efficiency can be obtained when using a tray operated according to Lewis' Case 2.
U.S. Pat. No. 5,223,183 to Monkelbaan, et al. teaches a parallel flow tray with at least one central downcomer and no side downcomers. The downcomers of each tray are aligned with the downcomers on the other trays such that the downcomers on one tray are immediately below those on the tray above. The outlets of one downcomer are directly above the inlet of another. A pair of inclined liquid deflecting baffles over each downcomer connects the outlets and inlets of vertically adjacent downcomers and provides a crisscrossing liquid flow path. The downcomer baffles prevent liquid from the tray above from entering each downcomer and define the direction of liquid flow onto the tray deck. The inclined surface of the baffle also imparts a horizontal momentum to the descending liquid which tends to push the liquid and froth present on the tray towards the inlet of the outlet downcomer for this portion or zone of the tray.
U.S. Pat. No. 5,318,732 to Monkelbaan, et al. teaches yet another parallel flow multiple downcomer type contacting tray, which increases tray capacity by providing imperforate stilling decks that extend across the tray deck surface outward from the downcomer inlet opening together with vertical inlet weirs attached to the outer end of the stilling decks. The inlet weirs may function as pre-weirs used in addition to the conventional inlet weir formed by the upward extension of the downcomer side wall. Further, the stilling decks help reduce pinching; however they also reduce the active area of the deck.
U.S. Pat. No. 7,204,427 to Xu et al. teaches still further parallel flow multiple downcomer type contacting trays. In the disclosed embodiments, side downcomers are incorporated into a parallel flow multiple downcomer tray having a center baffle and the downcomers have an inclined side wall that directs liquid onto the deck below. The inclined side wall also provides additional volume above the inferior downcomer inlet to reduce pinching at this inlet without the need for a stilling deck of the type disclosed in U.S. Pat. No. 5,318,732 to Monkelbaan, et al.
There is a need for improved high-capacity contacting trays that improve the overall mass transfer efficiencies by combining the features and benefits of a Lewis Case 2 parallel flow pattern and multiple pass tray configurations.